I got it working, goes to figure that I'd work it out myself after asking about it

If you want to try this on your Cub or any other A2A plane here's how.
(DONT FORGET TO BACKUP FIRST)
Browse to "Microsoft Flight Simulator X\SimObjects\Airplanes\A2A_PiperJ3\panel\a2a_piperj3"
Right click mapscreen.xml and choose edit, it will open in notepad.
Scroll down until you find
Code:
<CustomDrawParam id="ColorLayerFlightPlan" Name="ColorLayerFlightPlan">
<Value>0x7010B0</Value>
</CustomDrawParam>
</CustomDraw>
You need to insert a piece of code so it look like
Code:
<CustomDrawParam id="ColorLayerFlightPlan" Name="ColorLayerFlightPlan">
<Value>0x7010B0</Value>
</CustomDrawParam>
<CustomDrawParam id="TerrainShadow" Name="TerrainShadow">
<Value>1</Value>
</CustomDrawParam>
</CustomDraw>
Close and save the file and go test fly.
This should work for the maps in any of the A2A planes.
It also works for the default G1000, the TerrainShadow parameter already exists, you just need to set the <Value>0</Value> to <Value>1</Value>
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